Prepare the cookie dough layer: Place butter and sugar in a medium bowl and beat on medium speed with a handheld electric mixer until creamy. Beat in eggs, milk, and vanilla until smooth. In a separate small bowl, combine flour, baking soda, cornstarch, and salt. Beat flour mixture into liquid until mixture forms a dough. Stir in chocolate chips. Pat cookie dough evenly into the bottom of a foil-lined 13x9 baking sheet. Set aside.
Prepare the brownie layer: In a double-boiler over medium-high heat, melt butter and chocolate chips until smooth. Remove from heat and cool slightly. Meanwhile, whisk sugar, eggs, and vanilla until smooth. Gradually whisk melted chocolate mixture in until combined.
In a separate bowl, combine flour, cocoa, and salt. Gently fold flour mixture into liquid until just combined.
Pour brownie layer over cookie dough layer and gently smooth top. Bake brownies at 350F 30 minutes or until a toothpick comes out with moist crumbs sticking to it. Cool brownies in pan completely before cutting into squares.
Store brownies in an airtight container at room temperature up to 1 week.